Low-energy sodium hydroxide recovery for CO 2 capture from atmospheric air—Thermodynamic analysis Maryam Mahmoudkhani *, David W. Keith Energy and Environmental System Group, Institute for Sustainable Energy, Environment, Economy, University of Calgary, T2N 1N4 Calgary, AB, Canada 1. Introduction To avoid dangerous climate change, the growth of atmospheric concentrations of greenhouse gases must be halted, and the concentration may have to be reduced. The concentration of carbon dioxide, the most critical greenhouse gas, has increased from 280 ppm in the pre-industrial age to more than 380 ppm now and is now increasing by more than 2 ppm per year driven by global CO 2 emissions that are now increasing at more than 3.3% per year (Canadell et al., 2007). Carbon capture and storage (CCS) technologies target CO 2 removal from large fixed-point sources such as power plants. Stationary sources, however, emit approximately half of global CO 2 emissions. Direct capture of CO 2 from ambient air, ‘‘air capture’’, might be one of the few methods capable of systematically managing dispersed emissions. Therefore, while air capture is more expensive than capture from large point sources it remains important as it will primarily compete with emission reductions from dispersed sources such as transportation which can be very expensive to mitigate. The cost of air capture is uncertain and disputed. 1.1. Air capture Carbon dioxide absorption from atmospheric air using alkaline solution has been explored for half a century (Spector and Dodge, 1946; Tepe and Dodge, 1943) and was used commercially as a pre- treatment before cryogenic air separation. Large-scale scrubbing of CO 2 from ambient air was first suggested by Lackner et al. (1999) in the late 1990s. In wet scrubbing techniques, CO 2 is absorbed into a solution of sodium hydroxide, NaOH, and is leaving behind an aqueous solution of sodium hydroxide and sodium carbonate, Na 2 CO 3 . For this process, the contactor, as the component of the system that provides the contact between CO 2 and sodium hydroxide, has thus far been a point of contention. Large convective towers (Lackner et al., 1999), and packed scrubbing towers (Baciocchi et al., 2006; Zeman, 2007) have been the most frequently suggested designs. A packed tower equipped with Sulzer Mellapak has been investigated by Baciocchi et al. (2006) to absorb CO 2 from air with an inlet concentration of 500 ppm to an International Journal of Greenhouse Gas Control xxx (2009) xxx–xxx ARTICLE INFO Article history: Received 27 June 2008 Received in revised form 2 February 2009 Accepted 8 February 2009 Keywords: Air capture Sodium hydroxide Recovery Precipitation Direct causticization Titanate ABSTRACT To reduce the risks of climate change, atmospheric concentrations of greenhouse gases must be lowered. Direct capture of CO 2 from ambient air, ‘‘air capture’’, might be one of the few methods capable of systematically managing dispersed emissions. The most commonly proposed method for air capture is a wet scrubbing technique which absorbs CO 2 in an alkaline absorbent, i.e. sodium hydroxide producing an aqueous solution of sodium hydroxide and sodium carbonate. In most of the previous works it was assumed that the absorbent would be regenerated and CO 2 liberated from the alkaline carbonate solution using a lime and calcium carbonate causticization cycle. We describe a novel technique for recovering sodium hydroxide from an aqueous alkaline solution of sodium carbonate and present an end-to-end energy and exergy analysis. In the first step of the recovery process, anhydrous sodium carbonate is separated from the concentrated sodium hydroxide solution using a two-step precipitation and crystallization process. The anhydrous sodium carbonate is then causticized using sodium tri-titanate. The titanate direct causticization process has been of interest for the pulp and paper industry and has been tested at lab- and pilot-scale. In the causticization process, sodium hydroxide is regenerated and carbon dioxide is liberated as a pure stream, which is compressed for use or disposal. The technique requires 50% less high-grade heat than conventional causticization and the maximum temperature required is reduced by at least 50 8C. This titanate cycle may allow a substantial reduction in the overall cost of direct air capture. ß 2009 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +1 403 210 9137; fax: +1 403 210 3894. E-mail address: maryam@ucalgary.ca (M. Mahmoudkhani). G Model IJGGC-156; No of Pages 9 Please cite this article in press as: Mahmoudkhani M, Keith DW. Low-energy sodium hydroxide recovery for CO 2 capture from atmospheric air—Thermodynamic analysis, Int. J. Greenhouse Gas Control (2009), doi:10.1016/j.ijggc.2009.02.003 Contents lists available at ScienceDirect International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc 1750-5836/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2009.02.003